1. Field of the Invention
The present invention generally relates to a photodetector for optical communication and to a method of forming a p-i-n photodiode in a semiconductor.
2. Description of the Related Art
A p-i-n diode includes an intrinsic semiconductor layer (i.e., the i-region) that is sandwiched between p-type doped and n-type doped semiconductor layers. The low doping level in the intrinsic region causes this region to be depleted of carriers at low bias. When a reverse bias is applied to the p-i-n diode, most of the high electric field will develop in the intrinsic region.
When light is absorbed in the semiconductor, it produces electron-hole pairs. The high electric field serves to separate these carriers. Electrons are swept to the n-type region, where holes are swept to the p-type region. The photogenerated electrons and holes lead to current flow in the external circuit. This current is referred to as the xe2x80x9cphotogenerated currentxe2x80x9d, or photo-current.
For high speed operation, the intrinsic region is kept relatively thin (e.g., within a range of about 0.1 xcexcm to about 2 xcexcm) to reduce the transit time of the photogenerated carriers. For high quantum efficiency (e.g., defined as the number of electron-hole pairs generated per incident photons), the intrinsic region must be relatively thick so that most of the incident light would be absorbed (e.g., see S. M. Sze, Physics of Semiconductor Devices, 2nd Ed., Wiley New-York, 1981).
However, these two opposite requirements are more difficult to meet when the p-i-n photodiode is produced with an indirect bandgap material, where the absorption length is very large. xe2x80x9cAbsorption lengthxe2x80x9d is the length at which about 63% of the incident photons are absorbed. Thus, the absorption length is the typical length in which most (e.g., a predetermined high number) of the photons are absorbed. Such photons must be absorbed to become carriers. Generally, the thicker the material, the more absorption will occur. Hence, an absorption length/volume is required in which the photons can interact with a material (e.g., silicon in the case at hand) to be converted into carriers, which are in turn swept to electrodes (e.g., formed of doped polysilicon or the like).
An example of such an indirect bandgap semiconductor is silicon (Si), for which the absorption length is relatively large (e.g., about 15 to 20 microns for 850 nm light radiation) (e.g., see David F. Edwards, xe2x80x9cSilicon (Si),xe2x80x9d in Handbook of Optical Constants of Solids, ed. by Edward D. Palik, Academic Press, pp.547-568, (1985), and Geist, xe2x80x9cSilicon Revisited (1.1-3.1 eV),xe2x80x9d in Handbook of Optical Constants of Solids III, ed. by Edward D. Palik, Academic Press, p. 529, (1998)).
As a comparison with the indirect bandgap material, the absorption length is about 1 micron for a direct bandgap material such as GaAs (e.g., see Edward D. Palik, xe2x80x9cGallium Arsenide (GaAs)xe2x80x9d in Handbook of Optical Constants of Solids, ed. by Edward D. Palik, Academic Press, pp. 429-443, (1985)). Thus, a conventional silicon p-i-n photodiode is expected to have low speed operation or to exhibit low quantum efficiency at 850 nm.
Recently, a new p-i-n structure that decouples the absorption length from the photogenerated carrier transit time was proposed and experimentally tested (e.g., see U.S. Pat. No. 6,177,289, to J. Crow et. al, entitled xe2x80x9cLateral trench optical detectorxe2x80x9d). The structure includes alternating p-type and n-type parallel trenches separated by an intrinsic semiconductor. The structure forms a lateral p-i-n detector as illustrated in FIG. 1.
In the structure of FIG. 1, the photogenerated carriers only need to travel the short distance between two adjacent trenches before they are collected by the p-type and n-type plates. For this reason, the useful region for photon absorption is set by the depth of the trenches. Therefore, the trench depth should be comparable to the absorption length of the light in the semiconductor.
A monolithic integration of the p-i-n detector with the optical receiver electronics is very desirable since it can improve performance and reliability and reduce packaging costs. Since the receiver is fully realized by silicon technology, the photodetector must be made of silicon. Given that the absorption length in silicon is about 15 to 20 microns for 850 nm light radiation, to achieve high quantum efficiency trenches of comparable depth would have to be etched. Additionally, the trenches should be made as narrow as possible and take as little as possible of the surface real-estate, so that most of the detector surface is available for light absorption. However, these requirements impose the following problems.
First, most known etching processes can only yield a finite aspect ratio between the trench depth and width. For example, a high aspect ratio of 40:1 and a trench width of 0.2 microns yields a trench depth of about 8 microns. For silicon, this would be only about half of the required trench depth.
Even if the trench can be made deep enough, it may be difficult to fill with the conductor material. Most methods used in silicon manufacturing would form a seam in an attempt to fill the trench.
The deeper and narrower the trench is, the higher the series resistance it would have. This may affect the detector performance and result in a large RC time, where R is the trench series resistance and C is the device capacitance.
If the trenches are made shorter than the absorption length, then photons that are absorbed below the trench bottom generate carriers that drift slowly by following the weak fringing field between the n-type and p-type plates and eventually are collected by the plates. The weak field and the large diffusion length of carriers in silicon (about 80 microns) slow the detector response.
To eliminate carriers that are absorbed below the trench bottom line, an SOI trench photodetector was proposed (e.g., see U.S. patent application Ser. No. 09/678,315, to H. Kwark et. al, entitled xe2x80x9cSilicon-on-insulator trench photodiode structure for improved speed and differential isolation,xe2x80x9d having IBM docket number YOR920000052US1, and incorporated herein by reference).
The detector trenches are etched in a silicon layer that lays on an insulator film, as illustrated in FIG. 2. Photons that are absorbed below the insulator film are blocked by the insulator film and are not collected by the photodetector n-type and p-type plates.
However, the detector quantum efficiency is expected to be lower due to the loss of photons that are absorbed below the insulating film. Thus, the conventional structure of FIG. 2 is problematic as well.
Thus, in the above-described conventional methods and structures, some of the carriers are absorbed by the 8-micron electrodes (e.g., shown in FIG. 1) to produce a pulse, but other carriers are absorbed way beneath the 8-micron electrodes (e.g., shown in FIG. 1) in the substrate 1. As a result, these xe2x80x9cdeepxe2x80x9d carriers slowly drift to the electrodes to produce a pulse with a relatively long, long xe2x80x9ctailxe2x80x9d. This is problematic.
That is, ideally if a short light pulse is flashed, then the response of the photodetector is likewise a short current pulse corresponding to the short light pulse of the light source. However, as mentioned above, in the above situation where the carriers are drifting slowly toward the electrode from the substrate, such carriers produce the pulse with the long, long xe2x80x9ctailxe2x80x9d. Ideally, the pulse of current should correspond to/replicate as closely as possible the shape of the light pulse. The structure of FIG. 1 does not achieve such a replication.
Thus, the structure of FIG. 2 has been formed to have an insulator 2 to block the carriers which are absorbed deep in the silicon substrate 1, from slowly drifting up to the electrodes and there by prevent these carriers from contributing to the current being produced. However, this structure of FIG. 2 is problematic in its resistance, capacitance, as well as its efficiency, as described above.
In view of the foregoing and other problems, drawbacks, and disadvantages of the conventional methods and structures, an object of the present invention is to provide a new trench photodiode structure that is applicable to optical communication and is fully compatible with silicon processing.
Another object is to provide such a structure having a reduced junction capacitance and lower series resistance, exhibits a higher quantum efficiency, and allows efficient coupling of an optical fiber.
Yet another object of the present invention is to provide a method for the fabrication of the above trench photodiode.
In a first aspect of the present invention, a photodetector (and method for producing the same) includes a semiconductor substrate, a buried insulator formed on the substrate, a buried mirror formed on the buried insulator, a semiconductor-on-insulator (SOI) layer formed on the conductor, alternating n-type and p-type doped fingers formed in the semiconductor-on-insulator layer, and a backside contact to either one of the p-type doped fingers or the n-type doped fingers.
In a second aspect of the present invention, a photodetector (and method for producing the same) includes a semiconductor substrate, a buried insulator formed on the semiconductor substrate, a semiconductor-on-insulator (SOI) layer formed on the substrate, alternating n-type and p-type doped fingers formed in the SOI layer, a top mirror formed above the SOI layer, and a via formed in the semiconductor substrate for backside illumination, and for fiber alignment.
In a third aspect of the present invention, a method of forming a photo-detector, includes forming a buried insulator on a semiconductor substrate, forming a semiconductor-on-insulator (SOI) layer on the substrate, forming alternating n-type and p-type doped fingers in the SOI layer, forming a top mirror above the SOI layer, and forming a via in the semiconductor substrate for backside illumination, and for providing a fiber alignment cone.
In a fourth aspect of the present invention, a method of forming a photodetector having a fiber alignment cone, includes forming a semiconductor-on-insulator (SOI) layer over a substrate, forming alternating first-type and second-type trenches in the SOI layer, siliciding portions of the trenches, forming an insulator over the SOI layer, forming a mirror over the insulator, and etching a cavity in the substrate for receiving a fiber for backside illumination.
In a fifth aspect of the present invention, a complementary metal oxide (CMOS) chip, includes a semiconductor chip substrate, and a photodetector as described above, formed on the substrate.
With the unique and unobvious aspects of the present invention, a new trench photodiode structure (and method of forming the same) is provided which is applicable to light wavelength communication, and is fully compatible with silicon processing. The inventive structure has a reduced junction capacitance and lower series resistance, exhibits a higher quantum efficiency, and allows an efficient coupling of a fiber.